1. Field of the Invention
The present invention relates to a circuit arrangement for suppressing periodic or non-periodic noise voltage components superimposed by means of undesired influence on a DC signal to be evaluated as a line status signal in DC fed, two-lead transmission lines, particularly for the extension circuits in telephone systems fed via symmetrical branches in the subscriber circuit of the switching exchange and to be evaluated with respect to the changes in the status of the subscriber's loop.
2. Description of the Prior Art
For purposes of monitoring, it is necessary to determine the operating state of transmission lines. This is particularly true of extension circuits which carry a maximally allowable discharge current in the idle state and a minimum working current in the working state. The respective DC current value can be determined in the supply circuit assigned to each subscriber extension, since changes of potential which may be evaluated occur in this supply circuit given changes of operational status.
Noise signals can be superimposed on the useful signals to be evaluated, which noise signals, for example, arise due to inductive or capacitive coupling. These periodic or nonperiodic noise voltage components can influence the evaluation device in such a manner that erroneous evaluation results occur. Additional measures must be provided in order to obtain sufficient security against erroneous response.
By employing screening or, respectively, filter arrangements, in addition to a relatively high space requirement, there also occurs the disadvantage that such components can only be designed for a specific frequency or, respectively, for a specific frequency range. Furthermore, a complete elimination of the superimposed noise voltage components is not possible with such components.